I found out today that if I plug my Apple EarPods into my laptop's 3.5mm jack almost all the way, but not completely in, I can listen to music where the background music plays nearly perfectly, but I can barely hear the voices. Sometimes the voices become very clear, but only for a few seconds at a time. My computer's sound is perfectly operational; if I push the connector all the way in, I get normal-sounding sound.
It didn't matter whether I listened to the left or right EarPod; they sound exactly the same (to the limits of my perception -- but then again, I can't tell the difference between the left and right channels of audio anyway).
I was rather intrigued by this phenomenon, so I did some experimenting. I found some YouTube videos that played 20 Hz - 20 kHz tones in succession, and I couldn't hear anything. This was rather surprising to me, because music is just a Fourier series of pure sinusoids, and our hearing's bandwidth is limited to the range covered by the test.
I then hypothesized that I couldn't hear the test audio because the test video played the same thing for both left and right channels. Using a virtual audio cable, I changed the audio to 1 channel 24-bit audio, and sure enough, I couldn't hear anything in the music either.
On a limb, I decided to subtract the left channel from the right channel and listen to it. I followed the answer here, and generated a .wav with the two channels subtracted. It sounds exactly the same (with the headphones plugged all the way in) as what I heard with the connector not all the way in.
Why would this particular connection cause the left and right channels to be subtracted? I've built op-amp based voltage subtractors before, but I fail to see how a partly disconnected 3.5mm jack could emulate a voltage subtractor.
This is what my jack looks like. Normally, the white part is flush with the side of my computer.